Polyethylene have hitherto been developed as inexpensive engineering plastics such as high strength products, super high molecular weight products or the like, although it was impossible to coat or print paints or inks directly on them because of their non-polarity and extreme inertness and thus it was difficult to expand their application field in spite of their high performance and their low cost.
The high density polyethylene has hitherto been used in wide range of applications owing to its excellent physical properties and low cost. However, the high density polyethylene will be crystallized promptly by ultraviolet rays and deteriorated by crackings, so that it is not suitable for use in the open.
Therefore, polyethylenes reinforced with carbon or an anti-aging agent are placed on the market, but the powers of carbon or an anti-aging agent are limited and such a product has a black appearance and a limited use. Accordingly, a coating technique of polyethylene products has been researched for the purpose of obtaining an effective means for not only improving the appearance of articles by their decorative finishing but also extensively improving the field durability by screening ultraviolet rays.
However, polyethylene comprises absolutely a non-polar molecular structure and has no surface activity, so that it cannot be coated without special surface treatments.
As the surface treatment method which has hitherto been used most generally, there is a method in which the surface layer is oxidized with flame, arcing or a strong oxidizing agent and next coated with a topcoat.
However, such a treatment method is not suitable for practical use, since it a method in which it is difficult to maintain a constant adhesion due to the uneven treatment state and which is dangerous due to the high toxicity. As for the liquid primers for adhering polyethylene, the effectiveness of primers such as aminosilane, organic titanium, organic aluminum and the like is well known, but these primers are sensitive to humidity (and reactive rapidly with humidity) and it is required to coat thereon a topcoat (or an organic adhesive of a cyanoacrylate type) (or to apply an adhesive) within a very short time of a few minutes, so that they cannot be applied at all in use for a large area as in the case of paintings.
Furthermore, rubber requires additionally other properties depending on portions or time to be applied with the use of stretchability as an elastomer. As the synthetic rubbers, there are mentioned a variety of rubbers of a heat resistant type, an oil resistant type, a weatherable type, an energy saving type, a high functionalized special type and the like which correspond to these requirements. In particular, basic requirements for the tires of automobiles are safety and long-term durability, and molecules of a butadiene rubber, a styrene-butadiene rubber, an isoprene rubber and natural rubbers have been respectively designed according to the requirements for tires.
That is to say, one of the natural rubbers or a variety of synthetic rubbers is selected and incorporated with additives such as a vulcanizing agent, a vulcanization accelerator, an anti-aging agent, a reinforcing agent, a filler and a softener in consideration of their combination. For example, in order to improve weatherability, a rubber having fewer unsatisfied bonds is selected, and to the one having inferior vulcanizability are added the third components. In order to improve weatherability, carbon black may be added as a reinforcing agent.
However, these properties may be contrary to each other and often must be controlled by a compromise of each other.
Most of the synthetic rubbers have intrinsically unsaturated bonds, and thus they have essentially problems of weatherability, ozone resistance or the like. Therefore, these problems cannot be completely solved by designing the mixture of materials. The solution by admixture will always bring about secondary defects. For instance, the color of a material is limited to black by the addition of the carbon black, and such a limitation is not desirable given the colorful fashion of various life modes of the day.